Tens of thousands of protesters showed their anger at the Israeli bombing of Gaza today in a series of rallies across the UK that included throwing shoes at the gates of Downing Street.

More than a thousands pairs of footwear were thrown by protesters marching down Whitehall. A firework was also set off metres from the Downing Street gates.

The organisers said they wanted to leave shoes at the gates as a Muslim symbol of disgust at the attacks. When they were prevented from doing so, protesters began throwing their footwear, mimicking the Iraqi journalist who threw his shoes at George Bush at a press conference last month.

Protesters shouted: "Shame on you, have my shoe."

Shobhana Mukhi, from Beirut, said she had brought along a pair of trainers to throw because: "I wanted a physical sign of protest, people can talk about the numbers who attended, but if there's a physical representation of all of the shoes, then it sends a louder message."

Scotland Yard said its police officers were "not troubled" by the shoe throwing.

By 2pm the organisers said 50,000 were taking part and more were expected to join. A police spokesman put the attendance at 6,000 people as of 1pm, with more joining in.

The march departed from outside Embankment underground station shortly after midday, snaking its way along the banks of the river Thames before turning up Whitehall, past the entrance to Downing Street and reaching Trafalgar Square.

The crowd was a sea of Palestinian, Hamas and Hezbollah flags as well as the red standards of leftwing groups and placards denouncing the Israeli bombardment of Gaza.

Chants of "Stop, stop, Israel" and "Intifada now" rang out from the crowd, which was led by the veteran former Labour MP Tony Benn and leaders of British Muslim groups.

Ali Jamal, 25, a student from north London, said: "I find it absolutely shocking what is going on in Gaza and the fact that all leaders of the world are just standing back.

"The horror of the Israeli attack was summed up for me in the pictures of the three young sisters who were killed by a bomb which hit their house."

His friend and fellow student Ekram Ali, 20, said: "The reason I have come is to show Israel and the world that we don't tolerate what is happening.

"The British government just isn't doing enough and the media here are not telling people the true picture of what is happening."

Sarah Malin, 41, an actress from Brixton, had brought along her daughters Robin, three, and Evie, five, to show them what people were doing to stand up for whatthey believed was right.

"I don't come along to a lot of demonstrations like this but I wanted to on this occasion because what is happening in Gaza is just so unjust. I felt it was very important for my daughters to come and witness this. It's very emotive."

Aimee Shalan, 38, from Brixton and a member of the Council for Arab-British Understanding, said: "Palestinians have been suffering for decades now but I think on this occasion there is more British understanding about what is going on than there has been on previous occasions."

The crowds included many Muslims wearing headscarves as well as some wearing green headbands in solidarity with Hamas.

Some carried aloft baby dolls wrapped in Palestinian scarves and spattered with red paint to signify the deaths of Gazan children.

The march was supported by the comedian Alexei Sayle, the singer Annie Lennox, and the former mayor of London Ken Livingstone.

Sayle said he was speaking out because it was important for Jewish voice to be heard. "I want to feel proud of Israel, I want to be proud of my people but I am ashamed," he said at a press conference.

Lennox spoke of her shock at watching scenes of the bombing on television. "A few days after Christmas I came downstairs, put the television on, and saw smoke pyres coming from buildings and I was shocked to the core because I was thinking as a mother and as a human being: how was this going to be the solution to peace?"

Since Israeli air strikes started there have been daily protests outside the Israeli embassy in Kensington, west London, where large numbers have forced the closure of nearby streets. Today's demonstration in the capital was the biggest of at least 18 organised across the country.

In Portsmouth, nearly 500 people took to the streets in protest against the crisis. The protest began in the Guildhall Square and moved to a Tesco supermarket to call for a boycott on goods from Israel. Other rallies were taking place in Glasgow, Exeter, Edinburgh, Bristol, Liverpool, Norwich, Hull, Tunbridge Wells, Leeds, Manchester, Newcastle, Swansea, York, Caernarfon, Bradford and Sheffield.

• This article was amended on Monday 5 January 2009. Aimee Shalan is a member of the Council for Arab-British Understanding, not the Council for Our British Understanding. This has been corrected.